1. Field of the Invention
The present invention relates to a carrier for electrophotography used to develop an electrostatic image in electrophotography, electrostatic recording or electrostatic printing. It also relates to a two-component developer and an image forming method.
2. Related Background Art
It is conventionally known to form an image on the surface of a photoconductive material by an electrostatic means.
A large number of methods are known as electrophotography, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth. In general, an electrostatic latent image is formed on a photosensitive member, utilizing a photoconductive material and according to various means, and subsequently a very finely divided electrodetective material called a toner is adhered to the latent image to form a toner image corresponding to the electrostatic latent image. The toner is attracted to the electrostatic latent image in accordance with the quantity of charges on a photoconductive layer, so that a toner image with a difference in density is formed.
Next, the toner image is transferred to an image holding medium such as paper if necessary, followed by fixing by the action of heat, pressure, or solvent vapor. A copy is thus obtained. In the case when the process comprises a toner-image transfer step, the process is usually provided with the step of removing the toner remaining on the photosensitive member.
As developing methods by which the electrostatic latent image is formed into a visible image by the use of a toner, known methods can be exemplified by the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, and the method in which a conductive magnetic toner is used, as disclosed in U.S. Pat. No. 3,909,258, as well as what is called the J/B development as disclosed in Japanese Patent Application Laid-open No. 62-63970, in which a bias electric field comprised of an AC component and a DC component is applied across a developer carrying member (a developing sleeve) and a photoconductive layer to carry out development.
Among these, the magnetic brush development can be noted as a representative process. In this process, magnetic particles such as steel powder or ferrite powder are used as a carrier, and a developer comprised of a toner and such a magnetic carrier is held with a magnet so that the developer is arranged in the form of a brush by the action of a magnetic field of the magnet. The magnetic brush thus formed is brought into contact with the electrostatic latent image surface on a photoconductive layer, whereupon only the toner is attracted toward the electrostatic latent image from the brush to carry out development.
As toners used in these developing methods, a fine powder obtained by mixing and dispersing a colorant in a thermoplastic resin has been commonly used. The thermoplastic resin most commonly includes polystyrene resins. Besides, polyester resins, epoxy resins, acrylic resins and urethane resins are also used. As the colorant, carbon black is most widely used. In the case of magnetic toners, black magnetic powders of an iron oxide type are widely used. In a system in which what is called the two-component type developer is used, the toner is usually used by its mixture with carrier particles such as glass beads and iron powder.
The toner image finally formed on a copy image holding medium such as paper is permanently fixed onto the image holding medium by the action of heat and/or pressure. In this fixing, the step of fixing by heat has been hitherto widely used.
In the case when the process comprises a toner-image transfer step, the process is usually provided with the step of removing the toner remaining on the photosensitive member.
In recent years, a rapid progress is being made from monochromatic copying to full-color copying, and researches are made on two-color copying machines or full-color copying machines, which have been already put into practical use. For example, Journal of Electrophotographic Society, Vol. 22, No. 1 (1983) and Journal of Electrophotographic Society, Vol. 25, No. 1, p. 52 (1986) make reports relating to color reproduction and gradation reproduction.
Images formed by full-color electrophotography presently put into practical use, however, are not necessarily satisfactory for those who are accustomed to seeing color pictures that are by no means immediately compared with the actual object or original and also processed more beautifully than the actual object or original, as in television pictures, photographs and color prints.
Moreover, in recent years, there is an increasing commercial demand for making copying machines have a higher minuteness and making images have a higher quality. In the present technical field, it is attempted to make toner particle diameter smaller so that a color image can be formed in a high image quality. Making smaller the particle diameters of toner particles results in an increase in the surface area per unit weight, tending to bring about an excessively large quantity of triboelectricity of the toner. This is accompanied with a possibility of the insufficiency of image density or the deterioration of durability or running performance.
Namely, in the aforesaid development of electrostatic latent images, the toner is blended with a carrier formed of relatively large particles and is used as a developer for electrophotography. The composition of both the toner and the carrier is selected so that as a result of their mutual contact friction the toner can have a polarity reverse to the charges present on the photoconductive layer. As a result of contact friction between the both, the carrier electrostatically attracts the toner to its particle surfaces to transport the toner as a developer through a developing assembly and also feed the toner onto the photoconductive layer. When, however, copies are continuously taken on a large number of copy sheets by an electrophotographic copying apparatus using such a two-component type developer, although sharp images with a good image quality can be obtained at the initial stage, edge effect with much fog may seriously occur after copies have been taken on several tens of thousands of sheets, resulting in images having poor gradation and sharpness.
In color copying carried out using toners with chromatic colors, continuous gradation is an important factor that influences image quality, and the edge effect that stresses only margins of images, occurring after copies have been taken on a large number of copy sheets, greatly damages the gradation of images. For example, quasi-contours due to the edge effect are formed in the vicinity of actual contours, resulting in a loss of reproducibility including color reproducibility in color copying. Image area used in conventional black and white copying is 10% or less and images are almost held by line images as in letters, documents, reports and so forth. On the other hand, in the case of color copying, image area is 20% at least, and images are held by gradational solid images at a reasonable frequency or occupancy as in photographs, catalogues, maps, pictures and so forth.
When copies are continuously taken using such originals having a large image area, reproductions with a high image density can be obtained at the initial stage in usual instances, but the feeding of toner to the two-component type developer may become insufficient with time to cause a decrease in density, or the toner being fed and the carrier may mix in the state of charge insufficiency to cause fog or cause a local increase or decrease in toner concentration (which indicates toner-Carrier mixing ratio) on the developing sleeve, tending to result in bluffed images or non-uniform image density. This tendency becomes more remarkable when the toner has a smaller particle diameter.
Such under-development and fog are presumed to be caused by an excessively low toner content (i.e., toner concentration) in developer or a poor rise for rapid triboelectric charging between the toner being fed and the carrier contained in the two-component type developer, where any uncontrollable, insufficiently charged toner thereby produced participates in development. It is essential for color developers to have the ability to always output images with a good image quality in the continuous copying of originals having a large image area. To deal with originals having a large image area and requiring a very large toner consumption, measures hitherto taken have more relied on improvements of developing apparatus than improvements of developers themselves. That is, it has been attempted to increase the peripheral speed of a developing sleeve or make a developing sleeve have a larger diameter so that the developing sleeve can be brought into contact with electrostatic latent images more times.
Such measures can be effective for improving developability, but may greatly limit the lifetime of apparatus because of an in-machine contamination due to toner scatter from developing assemblies or because of an overload on the drive of developing assemblies. In some instances, measures are also taken in which developers are put in developing assemblies in large quantities in order to compensate the insufficiency of developability of the developers. Such measures, however, cause an increase in weight of copying machines, a cost increase due to the apparatus that must be made larger in size and an overload on the drive of developing assemblies as in the above case, and are not so much preferable.
Now, studies are reported on improvements made from both directions Of toners and carriers for the purpose of maintaining a high image quality over a long period of running.
More specifically, for the purpose of improving image quality, several developers are proposed. For example, Japanese Patent Application Laid-open No. 51-3244 discloses a non-magnetic toner in which its particle size distribution is controlled so that the image quality can be improved. This toner is mainly composed of toner particles having a particle diameter of 8 to 12 .mu.m, which are relatively coarse. According to studies made by the present inventors, it is difficult to "lay" the toner with such particle diameter onto latent images in a uniform and dense state, and also the toner, as having the feature that particles with a size of 5 .mu.m or smaller are in an amount of not more than 30% by number and particles with a size of 20 .mu.m or larger are in an amount of not more than 5% by number, tends to cause a lowering of uniformity because of a broadness of its particle size distribution. In order to form sharp images by the use of the toner comprised of such relatively coarse toner particles and having a broad particle size distribution, the toner particles must be thickly overlaid so that any spaces between toner particles can be filled up to increase apparent image density. This brings about the problem of an increase in the consumption of toner necessary to attain a given image density.
Japanese Patent Application Laid-open No. 54-72054 discloses a non-magnetic toner having a sharper particle size distribution than the above toner. This toner, however, contains medium-size particles with a size of as large as 8.5 to 11.5 .mu.m, and has room for further improvement for a toner with a high resolution.
Japanese Patent Application Laid-open No. 58-129437 discloses a non-magnetic toner having an average particle diameter of 6 to 10 .mu.m and held by particles with a size of 5 to 8 .mu.m in the greatest number. This toner, however, contains particles with a size of 5 .mu.m or smaller in an amount of as small as 15% by number or less, and tends to form images lacking in sharpness.
As a result of studies made by the present inventors, they have discovered that toner particles with a size of 5 .mu.m or smaller contribute the clear reproduction of contours of latent images and have a chief function of densely "laying" the toner onto the whole latent image. In particular, electrostatic latent images on a photosensitive member have a higher electric field intensity at their edges, the contours, than at their inner sides because of concentrated lines of electric force, and the quality of toner particles gathering at the contours influences the sharpness of image quality. The studies made by the present inventors have revealed that the control of the quantity of toner particles with a size of 5 .mu.m or smaller is effective for solving the problems concerning the sharpness of image quality.
Accordingly, the present inventors have proposed in Japanese Patent Application Laid-open No. 2-222966 a toner containing toner particles with a size of 5 .mu.m or smaller in an amount of 15 to 40% by number. This has brought about a reasonable improvement in image quality, but it is sought to achieve a more improved image quality.
Japanese Patent Application Laid-open No. 2-877 discloses a toner containing toner particles with a size of 5 .mu.m or smaller in an amount of 17 to 60% by number. This has certainly brought about stable image quality and image density, but it has been found that, when originals requiring a large toner consumption as in photograph originals are continuously copied, the particle size distribution of toner may change if measures are taken from the direction of toners only, making it difficult to obtain always stable images.
Meanwhile, Japanese Patent Applications Laid-open No. 51-3238, No. 58-144839 and No. 61-204646 suggest average particle diameter and particle size distribution of carriers. Of these, Japanese Patent Application Laid-open No. 51-3238 makes reference to a rough particle size distribution. It, however, has no specific disclosure as to magnetic properties closely concerned with developing performance of developers or transport performance thereof in developing apparatus. Moreover, carriers used in Examples all contain particles with a size of 250 meshes or larger in an amount of as large as about 80% by weight or more and also have an average particle diameter of 60 .mu.m or larger.
Japanese Patent Application Laid-open No. 58-144839 only discloses average particle diameter of a carrier. It does not make reference to the quantity of fine powder that influences the adhesion of carriers to photosensitive members and the quantity of coarse powder that influences the sharpness of images. It does not take account of performance of color copying, and has no detailed disclosure as to particle size distribution of carriers. As for Japanese Patent Application Laid-open No. 61-204646, it discloses as the gist of the invention a combination of a copying machine with a suitable developer, and has no specific disclosure as to the particle size distribution or magnetic properties of carriers. It also has no disclosure as to why the developer is effective for the copying machine.
Japanese Patent Application Laid-open No. 49-70630 has a disclosure relating to magnetic force of carriers, which, however, is concerned with iron powders used as carrier materials, having a larger specific gravity than ferrites, also having a high saturation magnetization. Iron powder carriers have been hitherto put into wide use, but tend to make the weight of copying macklines larger or cause an overload on drive torque, and also have a large environmental dependence.
A ferrite carrier disclosed in Japanese Patent Application Laid-open No. 58-23032 concerns a porous material with many voids. Such a carrier tends to cause the edge effect, having a poor durability, and has been found to be unsuitable for color copy carriers.
It has long been sought to provide a developer that enables continuous reproduction of images with a large image area, using a developer in a small quantity, and can satisfy the performance specific to color copying that no edge effect may occur even after running. Studies are made on developers and carriers, almost all of which, however, are proposed taking account of black and white copying, and only a little of which are proposed as those applicable also to full-color copying. It is also sought to provide a carrier having the ability to continue reproduction of images having an image area of 20% or more, which are nearly solid images, and having the ability to decrease the edge effect and retain the uniformity of image density on a sheet of reproduction.
Under such circumstances, the present inventors have proposed, as disclosed in Japanese Patent Application Laid-open No. 2-281280, a carrier with a narrow particle size distribution in which the presence of fine powder and the presence of coarse powder have been quantitatively controlled, to achieve a carrier improved in developing performance.
However, as previously stated, there is an increasing commercial demand for making copying machines have a higher minuteness and making images have a higher quality. In the present technical field, it is attempted to make toner particle diameter smaller so that a color image can be formed in a high image quality. Making smaller the particle diameters of toner particles results in an increase in the surface area per unit weight, tending to bring about an excessively large quantity of triboelectricity of the toner. This is accompanied with a possibility of the insufficiency of image density or the deterioration of running performance.
Thus, for the purpose of preventing the insufficiency of image density or the deterioration of running performance, caused by the toner made to have a smaller particle diameter, or for the purpose of improving development efficiency, it is attempted to make carrier particles have a smaller diameter. Such carriers, however, have achieved no quality high enough to stand against changes in the environment of toners or changes in the quantity of triboelectricity after running, and, under existing circumstances, it is difficult to achieve all the high image density, high image quality and good anti-fogging and prevention of carrier adhesion.